Hello
I try to follow the actual discussion with much interest and, I admit,
not full understanding. If my inputs are inappropriate, please feel free
to ignore this message.
I read the current spec and huge parts of today's discussions to find
out how images with multiple sources are intended to behave when
printed, or when the page is zoomed, but I found no hints. I think some
words on this might be useful in the spec, regardless of what the final
syntax will be.
1. Print
When a page is printed (or also converted to PDF or whatever), both
"viewport" width and pixel ratio change. Are UAs expected to load the
appropriate sources then? This could result in increased bandwidth,
delayed printing, and IMHO a disturbed user experience, as the image may
differ from the one seen on screen. Thus, I suggest to always use the
resource actually shown on screen for printing.
2. Zoom
On mobile devices, web pages are often zoomed out to fit the viewport
width by default, the user is supposed to manually zoom in and scroll in
order to read parts of pages. I understand that the whole thing about
responsive design is to make this kind of zooming unnecessary, but in
practice there will be all kinds of partly responsive designs using
responsive images.
Specially in cases where separate sources are given to match device
pixel densities, zooming might matter, as for a zoomed-out page the low
res image might be more than sufficient, but after zooming in the higher
resolution might be appropriate. Which OTOH can disturb the user
experience, when the images differ.